1. Field of the Invention
This invention relates to a device for preventing collisions between moving vehicles, especially between flying vehicles.
2. Description of the Related Art
The substantial loss of lives and property which generally occurs when aircraft collide with one another underscores the critical need for air traffic control (ATC).
Air traffic control (ATC) is currently accomplished by two methods.
The traditional technique utilizes a human being who observes a two-dimensional presentation of aircraft positions which have been obtained through radar surveillance.
If the aircraft are equipped with an electronic receiving and transmitting device known as a transponder, additional coded data appear on the two-dimensional display, i.e., the radar screen, near the light indicating the horizontal position of the aircraft. Such additional data are the four-digit code transmitted by the transponder and an identification of the aircraft determined by a ground-based computer which has been informed of the four-digit code which a human air traffic controller has assigned to the aircraft. Furthermore, some transponders transmit the altitude of the aircraft so that this information is displayed on the radar screen.
The human controller observes and determines, usually from an illuminated boundary on the radar screen, which aircraft are within the human controller's geographical area of responsibility. Should any two such aircraft come within a predetermined distance (considering both horizontal and vertical separation) of one another, they are considered to constitute "traffic" for one another. The human controller will then speak by radio to the pilot of one or both aircraft. In this voice radio message, the pilot will be advised at least of the range (distance) and horizontal bearing from his aircraft to the other aircraft; often, when such information is available, the pilot will also be told the altitude of the other aircraft.
It then becomes the pilot's responsibility vocally to acknowledge receipt of the voice radio message from the human air traffic controller; to attempt to make visual contact with the other aircraft, i.e., the traffic; to advise the human controller of the pilot's success in so doing; and, if necessary, to maneuver the pilot's aircraft to avoid a collision with the other aircraft.
The second method of air traffic control involves the utilization of collision avoidance systems located aboard aircraft and often also employing ground-based components.
The commercially pre-eminent version of collision avoidance systems is known as the Traffic alert and Collision Avoidance System (TCAS) and is described in U.S. Pat. No. 5,248,968:
"After two decades of development by the Federal Aviation Administration (FAA) and private contractors, TCAS has matured to a level where United States public law now requires that a TCAS be installed on commercial airplanes with more than thirty seats, starting in December of 1990."
"A TCAS-equipped airplane is surrounded by TCAS-protected airspace whose physical dimensions vary as a function of altitude and closure rate, i.e., the rate at which other airplanes are approaching the TCAS-equipped airplane. Being a time-based avionic system, TCAS continuously estimates and updates the flight paths of other airplanes through the interrogation of, and replies from, airborne radar beacon transponders located onboard the other airplanes. An airplane whose estimated flight path is projected to penetrate the TCAS-protected airspace is considered a collision threat (intruder) and annunciated to the flight crew of the TCAS-equipped airplane."
"TCAS-protected airspace can be divided into a caution area and a warning area, based on the estimated time to the Closest Point of Approach (CPA). About 40-45 seconds prior to CPA an intruder penetrates the caution area and causes the annunciation of a Traffic Advisory (TA). If the intruder continues to come closer to the TCAS-equipped airplane, at about 20-25 seconds to CPA, the intruder reaches the warning area, resulting in the annunciation of a Resolution Advisory (RA). Both TAs and RAs are constantly updated and, therefore, provide real time position and advisory information."
"TAs and RAs are annunciated both visually and aurally. The aural portion consists of voice messages. The visual portion of TA and RA annunciators includes a traffic display in the horizontal plane and, for RA annunciators, a resolution display in the vertical plane."
Unfortunately, TCAS necessitates placement on the aircraft of equipment that is too large, too heavy, and too expensive for the relatively small aircraft commonly used in general (non-airline) aviation.
Even smaller than the aircraft traditionally associated with general aviation are the unmanned terminology which has been established by the military and is used herein for consistency and clarity with previous publications, although the term pilotless would be more politically correct! aerial vehicles (UAV's) which the military has been employing with increased frequency since Operation Desert Storm to gather militarily significant data in the vicinity of an actual or potential battlefield without endangering a pilot and while presenting a smaller target for the enemy than does a traditional piloted military aircraft.
Just as the small general aviation aircraft create a potential hazard for one another, the UAV poses a significant potential hazard for the piloted military aircraft which necessarily must utilize the same airspace over a battlefield. Furthermore, the pilot may either be unaware of the potential presence of a UAV or may know only generally where one or more Equip may be encountered.
At least five patents cover collision avoidance systems or methods which depend upon the receipt of a transponder signal emitted by other aircraft. Transponders emit signals when they are interrogated by radar, specifically a secondary surveillance radar (SSR). None of these patents indicate whether the interrogating radar is airborne or ground-based, but none states that the interrogating radar is aboard the aircraft with the collision avoidance system.
U.S. Pat. No. 4,782,450 claims a method and device employing waves from a radar having a rotating beam and replies from a ground-based transponder to determine, through mathematical algorithms, the position of an aircraft equipped with special equipment and then utilizing a similar technique with the transponder replies from other aircraft to determine the positions of such other aircraft. The position of the ground-based transponder and of the radar must, according to lines 65 and 66 in column 8 of the patent, be stored in advance within the memory of the special equipment. This strongly suggests that the secondary surveillance radar will be on the ground.
The method and system of U.S. Pat. No. 5,075,694 utilize a direction-finding antenna to determine the direction of the interrogating source from the aircraft with the special equipment and, also, to determine the direction of another aircraft emitting a transponder reply. Additional techniques are then employed to determine the distance of the other aircraft from the specially equipped aircraft. But this method requires at least one "rotating interrogation signal source." The patent states, on lines 51 through 53 of column 4, that the system is designed to operate "in an environment having at least one rotating interrogation signal source (an SSR radar in the preferred embodiment)" and, on lines 5 through 7 of column 6, that "i!nformation" concerning each SSR signal source is acquired from an SSR database, suggesting that the secondary surveillance radar is ground based.
The system of U.S. Pat. No. 5,196,856 utilizes and improved method to determine the proximity of other transponder-equipped aircraft to a specially equipped aircraft based upon the time of arrival of transponder replies and the beam from the secondary surveillance radar interrogating such transponders. The patent, on lines 45 through 47 of column 12, indicates that the radar is ground-based by explaining that the radar is either "a rapidly rotating airport radar or a slowly rotating en route radar."
The pilot warning apparatus of U.S. Pat. No. 5,223,847 uses a directional antenna system on an aircraft for receiving transponder replies from other aircraft to determine bearing, a comparison of data from transponder Mode C (containing encoded data describing the altitude of the transponder-equipped other aircraft) replies with the altitude of the aircraft having the pilot warning apparatus, and a comparison of with prior received signals to determine whether the other aircraft and the aircraft with the pilot warning apparatus are coming closer to one another. In lines 34 through 39 of column 4, the patent indicates that the interrogating radar is ground based: "It is another object to provide a system for detecting potential midair collision threats from other aircraft without having to generate radio signals other than those that are already being generated by the equipment in the other aircraft in response to ground ATC air traffic control! interrogation."
Method claims in U.S. Pat. No. 5,157,615 determine a threat to a specially equipped aircraft by receiving replies from another aircraft with a transponder, learning the difference in altitude from the Mode C reply of the other aircraft and the altimeter of the specially equipped aircraft, and determining any closing trend from the strength of successive transponder replies. Although the patent does not state the origin of the interrogating signal, there is a strong inference that such interrogating signal originates away from the specially equipped aircraft because the patent, on lines 18 through 20 of column 37, describes the device employing the method as "a passive device performing effective proximity warning and collision avoidance functions" and, on lines 25 through 28 of column 37, asserts, "Notwithstanding its passive nature, . . . such device! monitors traffic in the vicinity of the host based on transponder relies sic! to SSR interrogations."
A direction-finding antenna system for receiving transponder replies from another aircraft accurately to determine the bearing of such other aircraft from the aircraft equipped with the direction-finding antenna system. The source of the interrogating signal is not identified.
None of the collision avoidance or warning systems discussed above, however, provide any information other than to the pilot of the aircraft equipped with the technology described in the patent. Protection of such other aircraft results, therefore, only indirectly from actions taken by the pilot of the aircraft equipped with the patented technology.
Moreover, to minimize the possibility of its being detected by the enemy, a UAV intentionally flies behind hills and in other locations that are inaccessible to coverage by air traffic control radar based either on the ground or in other aircraft, such as Airborne Warning and Command Systems (AWACS) aircraft. And general aviation aircraft without jet engines or turbocharged engines must, because of their somewhat limited service ceilings, when flying over mountains, fly so close to such mountains, that such general aviation aircraft are below the beams of air traffic control radar. Therefore, any collision avoidance system relying on interrogating radar that is not aboard the aircraft with the collision avoidance system would frequently be ineffective for such UAV's and general aviation aircraft.
The interrogating signal for the collision avoidance device of U.S. Pat. No. 4,161,729 emits its own interrogating signal. This signal is, however, detected only by other aircraft equipped with a similar collision avoidance device. The devices detect and compare Mode C altitude information from the transponder on both their own aircraft and that on the replying aircraft. The time to receive a return signal is detected; and if the other aircraft is within a given vertical distance, within a specified horizontal range, and approaching closer to the first aircraft, the collision avoidance device in each aircraft will alert the pilot of that aircraft.
Similarly, to work with another aircraft, the traffic monitoring device of U.S. Pat. No. 4,197,538 must be installed in both aircraft. The traffic monitoring device broadcasts, in code, the position and altitude of the aircraft in which it is installed. The identical device of the other craft then can display the position and altitude of the first aircraft and vice-versa. Unfortunately, not only must both aircraft be equipped with the traffic monitoring device, but broadcasting the position of one's aircraft may be extremely imprudent in a combat situation.
Identical disadvantages exist for the collision avoidance system of U.S. Pat. No. 5,153,836. The device of this system determines the position of the aircraft in which it has been installed, for example, by using GPS (Global Positioning System) or LORAN (Long Range Navigation) and then, just as did the traffic monitoring device of U.S. Pat. No. 4,197,538, broadcasts such position in a coded form which can be received and translated only by aircraft equipped with such a device. Of course, any aircraft that is equipped with this device will be able to have such device display the position of all other aircraft within range that are so equipped.
Finally, the anti-collision device of U.S. Pat. No. 4,104,638, when installed aboard an aircraft, provides information to other aircraft as long as such other aircraft have standard radio receivers and automatic direction finders (ADF's). The anti-collision device transmits tones. The ADF on a receiving aircraft then shows the bearing from the receiving aircraft to the aircraft with the anti-collision device. Proximity of the aircraft with the anti-collision device to the receiving aircraft can be inferred either from the rate of change of the ADF display or from the strength of the signal. Of course, to have any degree of precision, the measurement and comparison of signal strength would have to be accomplished by equipment that is not standard aboard an aircraft. Moreover, the aircraft with the anti-collision device obtains no information about other aircraft; and other aircraft do not learn the altitude of the aircraft with the anti-collision device.